The present invention relates to a process for forming polyesters and, more particularly, to a process for forming polyesters wherein catalysts are added.
At the present time, industrially-formed polyesters, in particular polyethylene terephthalates, are widely used for manufacturing fibers, films, and other industrial materials, in virtue of their high degree of crystallization, high softening point, and other various superior properties in terms of strength, chemical resistance, thermal resistance, weather resistance, electric insulation; etc. Conventional methods for forming such polyethylene terephthalates include, for example, a method for directly esterifying a terephthalic acid and an ethylene glycol by heating them at reaction temperatures ranging from 200.degree. C. to 280.degree. C. under atmosphere or pressure. Polyethylene terephthalates also can be formed by producing polymers, that is, bis (betahydroxyethyl) terephthalate and/or its low polymer (hereinafter, referred to as esterified compounds) by utilizing an ester interchange process that involves heating a dimethyl terephthalate and an ethylene glycol in the presence of catalysts at reaction temperatures ranging from 160.degree. C. to 240.degree. C., and continuously polycondensing the produced, esterified compounds in the presence of polymerization catalysts at reaction temperatures ranging from 260.degree. C. to 300.degree. C. in a high vacuum, in order to produce high polymers. Recently, the above-mentioned direct esterification method is mainly adopted to form industrial polyesters, since it is better than the latter ester interchange method, in terms of economical advantage.
Generally, reaction catalysts are used to accelerate and smoothly advance a reaction in producing polyesters. These catalysts include various compounds of metals, such as antimony, titanium, germanium, tin, zinc, manganese, lead; etc. It is well-known that the color and stability of the resulting polyester product and the reaction velocity are considerably varied depending upon the kinds of catalysts used. The reaction of forming polyesters is carried out in the presence of catalysts for a long time and at a high temperature. Accordingly, the reaction for forming polyesters of a high degree of polymerization in a short time is accompanied by several undesirable side reactions that result in coloring the polyester product yellow and increasing the content of diethylene glycol and the concentration of terminal carboxyl groups above their optimum levels. Consequently, the physical properties of the produced polyesters, for example, melting point, strength; etc., are deteriorated. Therefore, it is important to produce polyesters that can exhibit good color and superior physical properties even at a high reaction velocity. At the present time, an antimony compound, particularly antimony trioxide, is mainly used as an industrial polycondensation catalyst, since it is inexpensive and exhibits good catalytic activity and good thermal stability. However, antimony trioxide is hardly soluble in ethylene glycol or other reaction mixtures and tends to be precipitated during the reaction, thereby causing the resulting polyester to be colored gray or yellow-green or the transparency thereof to be decreased. These are more remarkable if the amount of the catalyst used and the reaction temperature are increased to improve productivity.
In order to provide catalysts solving the above-mentioned problems, there have been several methods proposed to reduce the esterification reaction time and the polycondensation reaction time and to produce polyesters exhibiting good color and superior physical properties. However, any one of them can hardly solve the above-mentioned problems. As a method for reducing the reaction time, there have been known, for example, a method that involves reacting a compound of silicone with a compound of titanium (U.S. Pat. No. 3,927,052), a method that involves dissolving antimony trioxide, a compound of cobalt and a compound of phosphorus in ethylene glycol (Japanese Laid-open Patent Publication No. Sho 53-51,295), and a method in which a compound of antimony is used with an organic acid (Japanese Laid-open Patent Publication No. Sho 60-166,320). However, these methods can not reduce both the esterification reaction time and the polycondensation reaction time. They also generate several problems in physical properties of the produced polyester in that the produced polymer is colored light yellow or the contents of diethylene glycol or terminal carboxyl groups are increased. On the other hand, as a method for improving the color and physical properties of the produced polymer, there have been known, for example, a method in which compounds of cobalt and alkali metal are used together with a compound of antimony (Japanese Laid-open Patent Publication No. Sho 58-117,218), a method in which a compound of antimony is used with a compound of tin (Japanese Patent Publication No. Sho 49-31,317), and a method in which antimony, tin, cobalt and alkali are used together with a compound of phosphorus(Japanese Laid-open Patent Publication No. Sho 62-265,324). However, these methods can not improve the color, transparency and physical properties of the produced polymer at the same time and provide any important advantage in terms of the reduction of the reaction time. As apparent from the above description, it is important to reduce both the esterification time and the polycondensation time and provide good color and superior physical properties of the resulting polymer, in order to obtain high quality polyesters with high productivity.